Even kids who know that ghosts aren’t real can get scared on Halloween.

Carole Pasquier/Wikipedia

Even if you’ve never been afraid of the dark, you might find yourself looking for monsters under the bed and sleeping with a night-light on as the end of October approaches. If so, don’t feel you’re acting like a coward. It’s perfectly normal to change your behavior when you feel afraid. After all, animals do it too.

“Fear matters,” says Karen Warkentin, a Boston University ecologist. “It’s a good thing,” she adds, “because fear makes you do things that keep you alive.”

Spooked frogs

Like kids, many animals experience fear. And they respond to the feeling in a variety of ways. Antelope on the plains of Africa, for example, run at the sight of a lion. A frightened turtle pulls its head and legs inside its shell. And small fish swim away when a big, hungry fish approaches.

Some animals respond to fear in ways you might not expect. The fear of being eaten, for instance, can scare some frogs right out of their eggs. Warkentin made that surprising discovery several years ago while studying tropical red-eyed treefrogs in Costa Rica.

The Costa Rican red-eyed tree frog lays its eggs in a jellylike mass on the undersides of leaves.

Karen Warkentin

In this species, female frogs attach jellylike clumps of their eggs to the undersides of leaves. The leaves hang on branches that dangle over ponds. When embryos hatch from the jellylike mass of eggs, tadpoles tumble into the water, where they eventually grow into adult frogs.

Treefrog eggs usually grow for 6 days before hatching. If the embryos sense that a hungry snake is about to attack, however, they can hatch up to 2 days ahead of schedule. Their snake predators can’t swim. So, by falling into the water early, the tadpoles escape the serpents’ hungry jaws.

How can unborn frogs know that a snake is about to attack? Warkentin looked for an answer to that question by experimenting with frog eggs in her laboratory.

A hungry cat-eyed snake attacks a sticky mass of tree frog eggs. To the right of the snake’s head, you can see a tadpole hatching early and escaping to the relative safety of a pond below.

Karen Warkentin

An approaching snake, she discovered, produces vibrations separated by brief pauses. When Warkentin shook a clump of frog eggs in a similar pattern, the embryos came tumbling out.

Other sound patterns, such as the pitter-patter of rain or the continuous shaking of a landing bird, didn’t seem to scare the embryos, and they stayed put. Warkentin concluded that treefrogs can both detect vibrations and recognize when the motion is coming from a snake.

If hatching early helps protect red-eyed treefrogs from snakes, you might wonder why their eggs don’t always hatch sooner. It turns out that hatching early brings its own dangers.

Once tadpoles land in the water, hungry fish, shrimp, and other animals like to eat them too. Staying in their eggs for a full 6 days, then, allows frog embryos to grow big and strong. This extra growth improves their odds of surviving in the water.

Tree frog embryos have plenty to fear, including hungry snakes on land and other predators in the water.

Karen Warkentin

In a snakefree environment, in other words, it makes the most sense for a frog embryo to wait 6 days before hatching. But, Warkentin points out, “If staying in the egg means you’re going to be eaten by a snake, you might as well find out what’s in the water.”

Ecology of fear

Fear can do more than affect animal behavior. It can actually influence entire ecosystems, say forest ecologists William Ripple and Robert Beschta of Oregon State University in Corvallis.

In Wyoming’s Yellowstone National Park, for example, the whole food web shifts when there are wolves around for the elk to worry about.

Fear of predators, such as this wolf in Yellowstone National Park, can cause an entire ecosystem to shift.

Barry O’Neill

Grey wolves eat elk. And there used to be lots of wolves in the park. Then, in the 1920s, wolves were removed from the area. As a result, the elk population exploded. Aspen trees in the park began to suffer because the large elk population was eating young trees before they had a chance to mature.

In the mid-1990s, scientists reintroduced wolves to the park. They expected that elk numbers would drop and aspen numbers would rise. Instead, a surprising sequence of events followed.

The elk population declined somewhat, but remained large enough to damage the aspens. Nevertheless, aspen trees returned to parts of the park where they hadn’t grown for years, Ripple and colleagues reported last summer.

So, how is it that young aspen trees are surviving, even as the elk population continues to thrive?

Ripple and Beschta suspect the park’s elk change their diets when they face an elevated risk (and fear) of being eaten themselves. Rather than munching on young aspen trees growing near streams, elk are spending more time on hills where they can easily see approaching wolves. Elk are also retreating deeper into the forest, where wolves are less likely to spot them.

Elk in Yellowstone National Park have a lot more to fear now that wolves are back.

NPS Photo by John Brandow

“With wolves back in the ecosystem,” Ripple says, “The elk become more wary, more vigilant, and much more careful as to where they go and where they browse and graze.”

He calls this ecosystem-wide phenomenon an “ecology of fear.” It’s possible, he says, that similar relationships shape food webs all over the world.

“There are probably many interconnections and functions that we don’t know about yet,” he adds.

It’s important that wildlife managers recognize that a predator like a wolf can affect plants, even when it doesn’t eat them, Ripple says. In Yellowstone, for example, it wasn’t necessary to kill elk in order to protect the aspens.

Sometimes, Warkentin says, with animals like these, “You just have to scare them.”

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“They’re so quick and so smart, it sometimes takes us hours with five people to catch one,” says Martin Wikelski. He’s an ecologist at Princeton University in New Jersey.

A dragonfly in flight.

iStockphoto.com

The pencil-thin, four-winged insects hover and dart, changing directions in an instant.

“They’re amazing,” Wikelski says. “They’re like little helicopters.”

Despite the challenges, scientists chase dragonflies to better understand their behavior, their habitats, and the dangers that threaten them. Surprises keep popping up, and dragonflies are giving researchers new ways to think about interactions in nature.

Long-distance flyer

Dragonflies date back at least 250 million years, says Daniel Soluk, an ecologist at the University of South Dakota in Vermillion. Alongside dinosaurs, they flitted across the prehistoric landscape on 2-foot-wide wingspans.

Although dragonflies have been around for a long time and live in many parts of the world, scientists still know remarkably little about them.

There are about 5,700 types of dragonflies around the world.

Photo by LaVonda Walton, U.S. Fish and Wildlife Service

You’ve probably seen dragonflies. The insects are welcome picnic guests because they eat mosquitoes. What you might not know is that some of these mosquito hunters are long-distance flyers.

Out of about 5,700 known species of dragonflies, as many as 50 species fly to warmer places for the winter, just like migrating birds do. Unlike birds, however, dragonflies appear to migrate in only one direction. Mom and dad may migrate south for the winter, but it’s the next generation that probably makes the return trip.

Radio alert

To confirm that certain dragonflies migrate, Wikelski and his coworkers used eyelash adhesive to attach tiny radio transmitters to individual insects. Even though the devices were very small, they still weighed about one-third as much as an adult dragonfly.

Luckily, dragonflies can carry heavy loads. “They seemed totally fine with it,” Wikelski says.

By attaching tiny radio transmitters to green darner dragonflies, researchers discovered that these insects can travel many kilometers in 1 day.

Photo © Phil Myers, Museum of Zoology, University of Michigan Animal Diversity Web site (animaldiversity.org).

In their experiment, Wikelski and his team used an airplane to track 14 dragonflies carrying transmitters. Twelve days of observations confirmed that the dragonflies did migrate.

When temperatures dropped two nights in a row, the researchers found, the dragonflies took off southward. In less than 2 weeks, some of the insects covered about 60 kilometers (37 miles).

Some dragonflies go even farther, Soluk says. They’ve been known to show up on ships hundreds of miles out at sea.

Mysterious flights

Strangely, no one has ever observed dragonflies flying north in the springtime, even though it must happen.

A female green darner dragonfly.

Photo by Michelle L. St. Sauveur, On Nature’s Wing Wildlife and Nature Photography (www.onnatureswing.com).

“Sometimes, dragonflies show up in New Jersey when ponds are still frozen,” Wikelski says. The insects had to have come from somewhere else because dragonfly eggs need liquid water to hatch.

“This is the kind of stuff that kids can observe,” he notes. By making such observations, students can help researchers understand where dragonflies come from and where they go.

Of the many remaining questions about dragonfly migration, Wikelski is particularly interested in finding out how much energy their journeys require. Evidence suggests that they simply ride the wind, so migration might be easy. He hopes someday to track dragonflies using satellites in space.

Puzzling patterns

Because of disappearing wetlands, the use of pesticides, and changes in groundwater flow, some dragonfly species are threatened with extinction. But scientists are discovering quirks in the behavior of these insects that might help keep them from dying out.

When Soluk started studying an endangered species called the Hine’s emerald dragonfly, for example, he was puzzled: Sometimes these dragonflies were easy to find, but at other times they were impossible to locate.

Hine’s emerald dragonfly has bright, emerald-green eyes and a metallic body, with yellow stripes on its sides. Its body is about 2.5 inches long and its wingspan reaches 3.3 inches. It is an endangered species and can be found today only in Illinois, Mich

U.S. Fish & Wildlife Service

For instance, when streams flowed in the spring, sampling turned up lots of Hine’s emeralds. As the water dried up, the dragonflies disappeared. Then, next spring, their population boomed again.

This species doesn’t migrate, so they were obviously hiding somewhere. But where?

“We spent a lot of time looking in places where we thought they might over-winter,” Soluk says.

After repeated failures, the researchers checked the last hiding spot available: crayfish burrows. Crayfish eat Hine’s emeralds, so it seemed an unlikely place for them to be.

“In the first burrow, we found two [dragonflies],” Soluk says. “In the second, we found 24. There were as many as 74 in one burrow.”

Saving endangered species

These findings suggest that saving an endangered species can be complicated. It would be easy, for example, to assume that getting rid of predators would help dragonflies, Soluk says. But in fact, Hine’s emeralds depend on crayfish burrows for their survival.

Crayfish eat dragonflies, but some dragonflies depend on crayfish burrows for shelter during the winter.

Photo by Eric Engbretson, U.S. Fish & Wildlife Service

It’s still not clear why crayfish don’t eat the dragonflies hiding in their burrows. It’s possible, for example, that the insects use camouflage or chemical odors to make themselves unappetizing.

Scientists are finding similar interactions among other creatures, including alligators and fish in Florida.

“Ecosystems are very complicated networks of interaction,” Soluk says. “We need to know all of the links before we start messing with them.”

The best way to protect dragonflies, Soluk says, is not to destroy their predators but to protect their habitats. With ever-expanding development and pollution, wetlands are disappearing, and dragonflies are losing sources of fresh groundwater.

By saving wetlands, we can save more than the Hine’s emerald. “They’re an indicator for a whole lot of other species that are vanishing,” Soluk says.

So, if you manage to catch a dragonfly, be gentle. Take a close look. Then, let it go. It has important work to do.

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